Image viewers in Linux are a dime a dozen. Some of these programs provide additional functions that make using a full-fledged image editing program unnecessary. gThumb [1] offers a particularly successful combination of image viewing and image editing for everyday use. It underwent a complete redesign together with numerous new functions on migration to Gtk3.

Since version 3.0.0, gThumb has used the Gtk3 toolkit from Gnome. The current version 3.4.1 works best in desktop environments such as Gnome or Cinnamon. When combined with other desktops, there may be problems with window dressing and widgets, but they can be resolved using a patch [2].

The Interface

The program welcomes you with an interface that needs some getting used to. It complies with the Gnome conventions introduced in version 3. In image editing mode, gThumb displays the current image file top left in the window, while a bar with previews of the remaining images in the active folder appears at the bottom across the width (Figure 1).

Figure 1: The gThumb interface provides an uncluttered view when started for the first time.

You will find various buttons arranged horizontally at the top right in the program window. The buttons can be used to activate various functions in the sidebar. With the introduction of client-side decorations in Gnome 3, the traditional menubar is gone, with just a few controls in the application window. These controls are usually provided by the window manager. This means, for example, that the buttons for minimizing, maximizing, and closing appear twice in other desktop environments, but this doesn't have any effect on the function. The individual pictograms on the buttons aren't always apparent at first glance. The software therefore reveals the function in a tool tip as soon as you mouse over a button.

Although you can change the view of the image or rotate it using the buttons at the top left, you will find the buttons for making modifications to the file on the right. The Properties button on the far left in this group lists the image's properties in a table. The one next to it, titled Edit File, branches into the editing menu: Here you can choose from seven functions in the Colors group and subsequently sharpen the content. The first function Automatic contrast adjustment appears particularly interesting: In our lab, this function significantly improved low-contrast photos of landscapes that appeared washed out because of strong sunlight and fog. This worked without having to reduce the brightness.

You can perform a complete adjustment using the next button to the right: As well as manually adjusting the brightness, contrast, saturation, and gamma values using slide controls, you can adjust the number of colors, although they correspond to the additive color model [3] by default. The third button Enhance Focus lets you sharpen an image. In doing so, the software displays a small section of the image as a preview so that you can see before editing whether visible artifacts appear in response to the restrictive settings. The Grayscale option, the fourth button, converts the image to black and white.

The second row of buttons in the Colors section lets you adjust colors and brightness levels as evenly as possible over the whole image with the use of histograms. The Special Effects tool has Instagram-like filters such as Vignette, Lomo, or Blurred Edges, which blurs an image's edges. The last button provides a function to correct red eye for images with which a flash was used.

Because gThumb performs all modifications immediately, you'll see straightaway whether the selected option positively affects the image quality. If it doesn't, you can undo the last action by clicking the back arrow button at the top left. gThumb provides various options for rotating and mirroring the displayed image in the Rotation button group. The Rotate function for free orientation automatically cuts the image upon request to the original format so that a larger number of images with slanted horizons can be corrected quickly.

The Format group provides functions for cropping and resizing an image. This way, you can use the mouse to change an image (e.g., if you want to use it full size for a postcard) to the necessary 3:2 form factor. The software places a grid over the active image during editing. With the mouse, you can move the grid to choose the section you want to use and change its size. You can save the changes to the disk using Save or Save As from among the buttons at the top in the right pane of the editing menu.

Search Function

If large image collections are stored on mass storage, managing the individual albums can become complex. However, gThumb doesn't just provide an option for searching for duplicates: It also simplifies the search for images by displaying the folder on the disk. You can show these by clicking the back arrow button with the arrow at the top left of the editing window titled View folders.

gThumb then opens a directory tree on the left in the program window and displays preview images from the active directory in a list in a large area on the right (Figure 2). If the open directory also contains videos, gThumb creates a preview for them too; however, this only works reliably with Gnome as the desktop environment. You can open it by simply clicking on an image, although gThumb does then immediately switch to editing mode.

Figure 2: You might have to comb through large collections of images for the right image in the folder view.

Searching by browsing through directories can be rather tedious with extensively nested directories. gThumb therefore has another search function which you can start by clicking the Find Files button at the top left of the window. This function enables a very specific search for individual images based on predetermined criteria, which you can select from a list and combine with each other.

In Stacks

gThumb also provides the option to edit image metadata. You can adjust the metadata according to your needs using a small dialog, which can be accessed via the Comment button at the top right of the editing window. In addition to comments and keywords in the General tab, in the Other tab, you can save copyright notices and store various information about the image.

gThumb also provides a function for completely deleting the metadata stored for each image when taking pictures using digital cameras. To use this function, activate the folder view and select the images whose metadata you want to delete from the preview. Then, click the Tools button with the wrench in the top right. The Delete Metadata entry then removes all additional information.

Particularly repetitive tasks, such as changing metadata or converting to another format, require a lot of work if you need to apply them to a variety of photos. gThumb therefore supports batch processing to handle such tasks automatically. This function can be used to rotate images and convert them to different formats and to modify image sizes, timestamps for comments, and last change dates.

You can access batch processing via preview mode. Simply select the images to be changed and open the Tools drop-down menu again (wrench symbol) in the folder view. gThumb performs options such as Rotate Left on all selected images without further prompting. However, other options such as Convert Format or Resize Images open a dialog with other settings.